فصلنامه رادار
سال نهم شماره 1 (پیاپی 25، بهار و تابستان 1400)

In this paper, phased multiple-input-multiple-output radars (known as PMRs) that transmitfrequency diverse orthogonal signals with full overlapped sub-arrays are studied. At first, theoptimal detector of PMR is designed by assuming heterogeneous clutter and random targetscattering coefficients. Then, for the optimal detector, the closed-form detection probability andfalse-alarm rate are computed. At the end, the power assigned to the orthogonal signals isoptimized analytically based on the convex optimization framework to maximize the detectionprobability. The numerical simulations show that the optimal detector is a joint spatial-temporalfilter that attenuates the clutters considerably by effectively combining orthogonal signals inorder to to improve the PMR detection probability in comparison with the phased radar (PR).Furthermore, simulation results illustrate that optimal power assignment in the form oforthogonal waves, based on the statistics of the target scatterings and that of the clutter,improves the detection performance of the PMR in comparison with the conventional equalpower assignment methods.

Numerous researches have been performed on the reflector antenna for realizing beam-shapingusing level-shaping. However, little study has been done for realizing the shaped beam throughthe multi-feed approach. The main goal of this paper is to provide a synthesis procedure forrealizing the flat-topped pattern in reflector antennas using the multi-feed approach. In this way,based on the genetic algorithm (GA) and utilizing the physical optic (PO) method, the excitationcoefficient of the feeding array is synthesized to achieve the desired pattern. Furthermore, in thispaper, an efficient method for reducing the computational time of PO integrals is introduced. Inthis way, the 3D PO integrals are mapped to 2D integrals; which results in about 30%computational time reduction. The proposed procedure is employed to realize a flat-toppedpattern in the angular scope of -20o to +20o and the radiation characteristics are compared withthose presented through the shaped reflector method in the literature. Results show that themulti-feed approach obtains the desired beam more accurately.

In this paper, the effect of errors in the Euler angles on the performance of the LEOsingle-satellite LOS geolocation method is analyzed. In the LEO single-satellite LOSgeolocation method, the aim is the estimation of the terrestrial target location by finding theintersection of the two direction vectors and the ground. In the presence of errors in themeasured values, the target position estimate deviates from its true value and it is required toanalyze the effects of this deviation. To this end, first, by expressing the problem in the ECEFcoordinate system and transferring the origin to the satellite position by using the latitude andlongitude of the satellite and the state vector, it is tried to determine the intersection of the LOSfrom the satellite to the target and the earth’s surface as the target location. Then, the effect ofthe error in Euler angles on the performance of the LOS method is theoretically analyzed and itscovariance matrix is derived by the perturbation method. In addition, the CRLB for the LOSmethod due to the error in Euler angles is presented. According to the simulation results, inorder to achieve an RMSE of less than 1500 meters in the considered scenario, the angle errorsin the attitude vector should be less than 0.1 degrees.

In this paper, the physical characteristics of a plasma reflector are investigated. It is observedthat the frequency and angle of the incident electromagnetic waves and also the physicalparameters of the plasma affect the characteristics of the plasma reflector. The analyticalanalysis show that a plasma reflector behaves like a high-pass filter whose cut-off frequency is afunction of the plasma frequency and the angle of the incident electromagnetic wave. Theamount of loss in this filter can be controlled by the plasma pressure and also the thickness ofthe reflector. It is possible to turn this filter on and off by energizing/de-energizing the plasma.In the case of the de-energized filter, the electromagnetic waves pass through it with a partialloss. In this research commercial off-the-shelf dielectric tubes which are cheap and available areused to implement the concept, and various feasible arrangements, for the implemetation of thereflectors with cylindrical dielectric tubes (equivalent to commercial plasma containers) areinvestigated. The simulation results show that using two tublar layers perpendicular to eachother, is the most efficient structure for the plasma reflector.

As underground hidden facilities are used for various applications, imaging systems are requiredfor the detection and recognition of these facilities. In this paper, imaging by mechanical wavepropagation in the soil is investigated. Most underground facilities can be considered as airchambers located in the middle of the soil. Air and soil have a large difference in acousticimpedance, so underground facilities can produce a large reflective signal because the amplitudeof the reflected signal depends on the difference in impedance of the two materials. Seismic andnon-destructive concrete testing systems also use the process of propagation of mechanicalwaves in the material. Although seismic systems have been successful in the detection issue,they require several meters of space for equipment layout, and transportation complexity is alsoone of their problems. They are unsuitable for urban space because they are inherently designedto detect water sources at depths of several hundred meters. On the other hand, while thenon-destructive concrete testing system equipment have suitable dimensions, they have lowpenetration depth which is impractical for this purpose. Seismic and non-destructive testingsystems of concrete operate in the range of subsonic and ultrasonic waves, respectively. Thisresearch project proposes the idea that by choosing an operating frequency between the twomentioned ranges, it is possible to obtain equipment with the appropriate dimensions forimaging underground facilities. In this project we managed to image an undergroundconstructed cylindrical cavity with the diameter of 1 meter and the depth of 4 meters. Theaccuracy of this method depends on the sound propagation speed in the material. Since there areaccurate relationships for the propagation speed of sound in air, the diameter of the cylinder wasestimated with an accuracy of about 4%, but with the measurement of propagation speed beinginfeasible in this project, leading to the ambiguity in the value of propagation speed of sound insoil, the values cited in several reference sites were used, giving the accuracy of 4 to 20%.

In this paper, the scattering of long wavelength electromagnetic waves from a warm plasma column with an elliptical cross section is investigated. We assume that the incident wavelength is much larger than the cross-sectional area of the elliptical column and enter the thermal effects into the calculations using the Boehm-Gross equation. Using suitable boundary conditions, we calculate the electric potential as well as the electric field at the points inside and outside the plasma column with an elliptical cross section. Then we obtain the dispersion equation in this case and show that in the limit state, this equation becomes the dispersion equation in a plasma column with a circular cross section. Considering the plasma resonance condition in an elliptical cross-section plasma column, we calculate the resonant frequency in this configuration. We plot figures of potential amplitude at the inside and outside points. Note that using the resonant frequency, we can calculate the characteristics of the plasma antenna under study, such as plasma density.

Radar network deception through phantom track generation using cooperative unmanned aerialvehicles (UAVs) is an effective way of encountering sophisticated radar networks. In previousstudies a method based on virtual motion camouflage (VMC) and path control parameter (PCP)has been developed to design the trajectory of cooperative UAVs during the phantom trackgeneration mission. In these studies, the radar locations are assumed to be known. In realsituations, the knowledge of radar locations has an inaccuracy. The radar position inaccuracyleads to an incoherent phantom track. The radar network can recognize the phantom trackaccording to the distance between the false targets created by each UAV. In this paper, the effectof the inaccuracies of the radar position on the generated phantom track is considered. Byadding a constraint that limits the flyable range of UAVs, the phantom track is generated suchthat the probability of the radar network detecting the deception is minimized. The mentionedconstraint is specified based on the maximum radar position inaccuracy, the radar error, and thefalse target trajectory. The simulation results show that using this constraint in the trajectorydesign for UAVs, reduces effectively the recognition rate of the phantom track for variousranges of inaccuracies.

This paper presents the design of a high-isolation power divider based on the groove gapwaveguide. The proposed structure is similar to a rectangular waveguide Magic-T when itsdifference port is terminated using a waveguide match-load. Wideband impedance matching andhigh-isolation of the structure are obtained by placing some PEC steped cones inside thestructure. First, a 1×2-way power divider is designed such that by combining a specific numberof it, power dividers with more outputs can be acheived and to demonstrate this idea a 1×4-waypower divider is assembled. The results of simulating the proposed power divider show that thestructure has 33% frequency bandwidth covering from 42 GHz to 59 GHz. The structurereflection coefficient is below -15 dB, the insertion loss is lower than 0.12 dB and the isolationis better than 18 dB in the whole frequency bandwidth and the simulation results prove that theproposed power divider is an excellent candidate for millimeter-wave applications.

The phased array radars are a favourable type in the group of radars due to their agility inscanning the space and their highly flexible radiation patterns. However, due to theirdependence on the phase to frequency shifters, these radars suffer from “beam squint”, whichlimits the radar bandwidth. Using the true time delay (TTD) technique is one of the suggestedsolutions to this problem. Due to their unique features, the optical circuits have been widelyused for the implementation of the TTD method but these circuits have problems such as highcomplexity, high cost, and the large size of structures. For example, fiber delay lines can onlygenerate a certain and limited amount of delay, they are difficult to maintain, and they take up alot of space. Conversely, the delay lines based on ring resonators create continuous delay andoccupy little volume. However, because of the very small size of the rings, these lines face thechallenge of complexity in construction. In this paper, a delay line based on ring resonators andmicrocomb laser source, with a new structure is presented that while enjoying the conventionaladvantages of these rings, is simpler in structure and requires fewer rings than otherconventional configurations.

In a location estimation system, the deployment of reference nodes significantly affects theaccuracy of estimating the location of the desired points. In this paper, it is assumed that thelocation system is equipped with a number of reference nodes in a specified arrangement, andthe goal is to add a number of new reference nodes to improve the location estimation accuracyof a desired emitting node. In this network, the location of an emitting node is estimated bymeasuring the distances to the reference nodes. An algorithm is proposed to find the optimalplace of the new added reference nodes using the frame theory. The basis of this algorithm is toarrange the new reference nodes without changing the location of the present reference nodes inthe system in such a way that the determinant of the Fisher information matrix is maximized.The simulation results verify the efficiency of the proposed algorithm in the optimal deploymentof new reference nodes.

The synthetic aperture radar is an imaging radar that has a high resolution. The syntheticaperture radar image may be degraded by the interference of radio frequencies and anincomprehensible image may be created. Interferences in the synthetic aperture radars aredivided into the three categories of , , and , which represent radio frequency noise interference,narrow band interference and wideband interference, respectively. To effectively reduce theinterference in synthetic aperture radar images, first the presence of interference and its typeshould be asserted and then the interference reduction algorithms should be calculated accordingto interference type. In this paper an algorithm for the detection of interference and its type inthe synthetic aperture radar images is presented. Whilst in the previous articles the SSD methodis used for interference detection, in this paper we have used the Faster RCNN method based onneural network convolutional which has a higher speed and accuracy than the SSD method. Inthis method, first a neural network is trained with the ability of multiple classification. Then theFaster RCNN is constructed with the neural network and and is trained by 25 time - frequencyimages from the artificial aperture radar signal. The trained network is able to detect anyinterference in the radar signal of a synthetic window with 99% accuracy. After detecting theinterference by the proposed algorithm, the normalized least mean square filter is able to reducethe interference and improve the radar image. This filter operates similarly in decreasing allthree types of interference.

In this paper a modified Gaussian pulse stimulus is employed to improve the accuracy of tumordetection inside breast phantom for 2D reconstructed image results in cylindrical setup of themicrowave imaging system. This pulse shaping puts more energy at higher frequencies incontrast with conventional Gaussian pulse shaping in the impulse radar. Hence a widerbandwidth is available to achieve higher accuracy for precise spatial localization. In the presentarticle a simulated cylindrical setup of the microwave imaging system with a modifiedstimulated pulse is generated. The main purpose of this paper is employing a new stimulatingpulse to detect a tumor from a biological phantom for 2D visualization of time-domain results.In order to achieve the goal, the advantages of generating a novel confocal image-reconstructingalgorithm based on back-projection method is employed. The advantage conferred by “highresolution imaging” is that more energy is used at reflected signal than with conventionalconfocal imaging, and subsequently a relatively lower spatial resolution in identifying thereflected signal is achieved. Simulated results are presented to validate the effectiveness of theproposed method for precisely calculating the time-dependent location of targets.